/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

package org.apache.iotdb.tsfile.encoding.encoder;

import java.io.ByteArrayOutputStream;

import static org.apache.iotdb.tsfile.common.conf.TSFileConfig.*;

/**
 * This class includes code modified from Michael Burman's gorilla-tsc project.
 *
 * <p>Copyright: 2016-2018 Michael Burman and/or other contributors
 *
 * <p>Project page: https://github.com/burmanm/gorilla-tsc
 *
 * <p>License: http://www.apache.org/licenses/LICENSE-2.0
 */
public class LongGorillaEncoder extends GorillaEncoderV2 {

    private static final int ONE_ITEM_MAX_SIZE =
            (2
                    + LEADING_ZERO_BITS_LENGTH_64BIT
                    + MEANINGFUL_XOR_BITS_LENGTH_64BIT
                    + VALUE_BITS_LENGTH_64BIT)
                    / Byte.SIZE
                    + 1;

    private long storedValue = 0;

    @Override
    public final int getOneItemMaxSize() {
        return ONE_ITEM_MAX_SIZE;
    }

    @Override
    public final void encode(long value, ByteArrayOutputStream out) {
        if (firstValueWasWritten) {
            compressValue(value, out);
        } else {
            writeFirst(value, out);
            firstValueWasWritten = true;
        }
    }

    @Override
    public void flush(ByteArrayOutputStream out) {
        // ending stream
        encode(GORILLA_ENCODING_ENDING_LONG, out);

        // flip the byte no matter it is empty or not
        // the empty ending byte is necessary when decoding
        bitsLeft = 0;
        flipByte(out);

        // the encoder may be reused, so let us reset it
        reset();
    }

    @Override
    protected void reset() {
        super.reset();
        storedValue = 0;
    }

    private void writeFirst(long value, ByteArrayOutputStream out) {
        storedValue = value;
        writeBits(value, VALUE_BITS_LENGTH_64BIT, out);
    }

    private void compressValue(long value, ByteArrayOutputStream out) {
        long xor = storedValue ^ value;
        storedValue = value;

        if (xor == 0) {
            skipBit(out);
        } else {
            writeBit(out);

            int leadingZeros = Long.numberOfLeadingZeros(xor);
            int trailingZeros = Long.numberOfTrailingZeros(xor);
            if (leadingZeros >= storedLeadingZeros && trailingZeros >= storedTrailingZeros) {
                writeExistingLeading(xor, out);
            } else {
                writeNewLeading(xor, leadingZeros, trailingZeros, out);
            }
        }
    }

    /**
     * If there at least as many leading zeros and as many trailing zeros as previous value, control
     * bit = 0 (type a)
     *
     * <p>store the meaningful XORed value
     *
     * @param xor XOR between previous value and current
     */
    private void writeExistingLeading(long xor, ByteArrayOutputStream out) {
        skipBit(out);

        int significantBits = VALUE_BITS_LENGTH_64BIT - storedLeadingZeros - storedTrailingZeros;
        writeBits(xor >>> storedTrailingZeros, significantBits, out);
    }

    /**
     * Stores the length of the number of leading zeros in the next 6 bits
     *
     * <p>Stores the length of the meaningful XORed value in the next 6 bits
     *
     * <p>Stores the meaningful bits of the XORed value
     *
     * <p>(type b)
     *
     * @param xor           XOR between previous value and current
     * @param leadingZeros  New leading zeros
     * @param trailingZeros New trailing zeros
     */
    private void writeNewLeading(
            long xor, int leadingZeros, int trailingZeros, ByteArrayOutputStream out) {
        writeBit(out);

        int significantBits = VALUE_BITS_LENGTH_64BIT - leadingZeros - trailingZeros;

        // Number of leading zeros in the next 6 bits
        // Different from original, maximum number of leadingZeros is stored with 6 (original 5) bits to
        // allow up to 63 leading zeros, which are necessary when storing long values
        // Note that in this method the number of leading zeros won't be 64
        writeBits(leadingZeros, LEADING_ZERO_BITS_LENGTH_64BIT, out);
        // Length of meaningful bits in the next 6 bits
        // Note that in this method the number of meaningful bits is always positive and could be 64,
        // so we have to use (significantBits - 1) in storage
        writeBits((long) significantBits - 1, MEANINGFUL_XOR_BITS_LENGTH_64BIT, out);
        // Store the meaningful bits of XOR
        writeBits(xor >>> trailingZeros, significantBits, out);

        storedLeadingZeros = leadingZeros;
        storedTrailingZeros = trailingZeros;
    }
}
